Fluke PM6690 - How to Use Trigger Level Setting

trigger or hysteresis window more than once
per input cycle and give erroneous counts.
Fig. 3-13 shows that less noise still affects the
trigger point by advancing or delaying it, but
it does not cause erroneous counts. This trig-
ger uncertainty is of particular importance
when measuring low frequency signals, since
the signal slew rate (in V/s) is low for LF sig-
nals. To reduce the trigger uncertainty, it is de-
sirable to cross the hysteresis band as fast as
possible.
Fig. 3-14 shows that a high amplitude signal
passes the hysteresis faster than a low ampli
-
tude signal. For low frequency measurements
where the trigger uncertainty is of importance,
do not attenuate the signal too much, and set
the sensitivity of the counter high.
In practice however, trigger errors caused by
erroneous counts (Fig. 3-10 and Fig. 3-12) are
much more important and require just the op
-
posite measures to be taken.
To avoid erroneous counting caused by spuri
-
ous signals, you need to avoid excessive input
signal amplitudes. This is particularly valid
when measuring on high impedance circuitry
and when using 1MW input impedance. Under
these conditions, the cables easily pick up
noise.
External attenuation and the internal 10x
attenuator reduce the signal amplitude, includ
-
ing the noise, while the internal sensitivity
control in the counter reduces the counter’s
sensitivity, including sensitivity to noise. Re-
duce excessive signal amplitudes with the 10x
attenuator, or with an external coaxial
attenuator, or a 10:1 probe.
How to use Trigger Level
Setting
For most frequency measurements, the
optimal triggering is obtained by positioning
the mean trigger level at mid amplitude, using
either a narrow or a wide hysteresis band, de
-
pending on the signal characteristics.
When measuring LF sine wave signals with
little noise, you may want to measure with a
How to Reduce or Ignore Noise and Interference 3-7
Input Signal Conditioning
Fig. 3-14 Low amplitude delays the trig
-
ger point
Fig. 3-15 Timing error due to slew rate.
Fig. 3-13 Trigger uncertainty due to noise.